Isotachophoresis (ITP) is a preconcentration and separation technique which leverages electrolytes with different electrophoretic mobility to focus (and in some cases, separate) ionic analytes into distinct zones. In ITP, analytes simultaneously focus and separate between high effective mobility leading electrolyte (LE) ions and low effective mobility trailing electrolyte (TE) ions. The balance of electromigration and diffusion at the zone boundaries in ITP results in sharp moving boundaries which can be described as ion concentration shock waves. It is convenient to refer to such boundaries as “shock waves”, “shocks” or “ITP shocks” for brevity. We note that “shock wave” is a generic term which has been used in the literature to describe self-sharpening discontinuities due to non-linear advection flux in a variety of physical processes, such as fluid flow, chromatography, ITP, sedimentation boundaries, and automotive traffic flow.
Typically, ITP experiments are performed separately for focusing anions or cations in respectively anionic and cationic ITP. However, anionic and cationic ITP can also be performed simultaneously in a single channel. The latter approach, called bidirectional ITP, has been used to provide simultaneous separation of anions and cations with ITP. An article by Prest et al. (Bidirectional isotachophoresis on a planar chip with integrated conductivity detection, Analyst, 2002, 127, pp. 1413-1419) is an example of this approach.